Science Behind Mysterious 'Fifth Taste' Revealed

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Science Behind Mysterious 'Fifth Taste' Revealed

It's appetizing news for anyone who's ever wanted the savory taste of meats and cheeses without actually having to eat them: chemists have identified molecular mechanisms underlying the sensation of umami, also known as the fifth taste.

The much-loved but historically unappreciated taste is produced by two interacting sets of molecules, each of which is needed to trigger cellular receptors on a tongue's surface.

"This opens the door to designing better, more potent and more selective umami enhancers," said Xiaodong Li, a chemist at San Diego-based food-additive company Senomyx. Li co-authored the study, published Monday in the Proceedings of the National Academy of Sciences.

Four other basic tastes — bitter, sweet, salty and sour — were identified 2,400 years ago by the Greek philosopher Democritus, and became central to the western gastronomic canon.

In the late 19th century, French chef and veal-stock inventor Auguste Escoffier suggested that a fifth taste was responsible for his mouth-watering brew. Though Escoffier's dishes were popular, his theories were dismissed until 1908, when Japanese chemist Kikunae Ikeda showed that an amino acid called glutamate underlies the taste of a hearty variety of seaweed soup.

In honor of Ikeda, the taste was dubbed umami, the Japanese word for delicious. It took another 80 years for umami to be recognized by science as comparable to the other four tastes.

In the meantime, monosodium glutamate became wildly popular as a flavor enhancer. But MSG can cause headaches and dizziness, and has been tenuously linked to long-term neurological disorders. Between the public dissatisfaction with MSG and growing demand for artificial meats and dairy products, an umami alternative is welcome.

"The only way to have a substitute is to find the molecular target of glutamate. If we figure that out, then we can screen for agents that are not glutamate but could mimic it," said Johns Hopkins University neuroscientist Solomon Snyder, who was not involved in the new study.

Li's team took human kidney cells and added the genes for receptors linked to umami taste. Receptors formed on the cells' surface, geometrically resembling the mouth of a Venus flytrap — the plant renowned, appropriately, for its meat-eating proclivities.

When glutamate caught on a receptor's lips and a molecule called ribonucleotide lodged in its throat, the receptor snapped shut.

"The configuration of the receptor changes, sending a signal down into the cell," said Li. In their engineered and disconnected cells the signal quickly fizzled — but in a tongue surface cell, said Li, "Your brain gets a signal: Something tastes good that is in my mouth."

The umami receptor's shape is similar to that of sweetness receptors, he said, and his team's research could eventually suggest alternatives to sugar. But more work is needed to determine exactly what happens when signals are sent from tongue to brain.

"Right now we're working on the first step of the activation of this receptor. Downstream, there are still lots of things to understand,"said Li, who added that his favorite umami-containing food is lobster.